Ethylene cracking technologies used in ethylene plants mainly include those developed by LUMMUS Co. (USA), Stone & Webster Co. (USA), Kellog & Braun Root Co. (USA), Linde Co. (Germany), and Technip KTI Co. (France/Netherlands), and CBL cracking furnace technology developed by China Petroleum & Chemical Corporation.
FIG. 1 shows a prior art ethylene cracking furnace, which includes a radiant section 1, a convection section 3, and a flue section 2 located between the radiant section 1 and the convection section 3. Within radiant section 1, a radiant coil 4 is arranged in the central plane P of radiant section 1 along the longitudinal direction thereof. In addition, radiant section 1 is further provided with bottom burners 5 and/or side burners 6 for firing. Moreover, the ethylene cracking furnace further includes transfer line exchanger(s) 7, a high-pressure steam drum 8 and an induced draft fan 9.
The radiant coil 4 generally may have a single-pass, two-pass or multi-pass structure. Among these structures, the single-pass radiant coil has the best cracking selectivity since it has the largest specific surface area, the shortest resident time, the highest cracking temperature, and a quick temperature increase. However, the single-pass radiant coil suffers from a large coking speed and a short run length. Moreover, since the outlet end of the single-pass radiant coil is directly connected to the transfer line exchanger's inlet, the radiant coil has to expand downwardly. The resulting thermal stress is difficult to overcome.
To significantly reduce feedstock consumption, maintain a suitable run length, and have a good feedstock flexibility, two-pass radiant coils having two pass tubes are used for liquid feedstock in most cracking furnace technologies. The first pass tube may have a small diameter. Therefore, a quick temperature increase can be achieved through the relatively large specific surface area of the small-diameter tube. The second pass tube may have a large diameter which reduces the influence on coking sensitivity. However, with respect to the two-pass radiant coil, the tube wall temperature of the first-pass tube is different from that of the second-pass tube, and therefore the thermal stress generated in the two pass tubes are different from each other. Consequently, the lifetime of radiant coils in two pass radiant coils are easily shortened. Multi-pass radiant coils are used for gas feedstock in most cracking furnace technologies. However, the tube wall temperature in a one pass tube is different from that in other pass tubes. Thermal stress generated in different pass tubes of multi-pass radiant coils are different from each other. Consequently, the lifetime of radiant coils will be also easily shortened. Additionally, two-pass or multi-pass radiant coils, including a plurality of tubes occupy a relatively large space in the furnace, requiring an increased size of the cracking furnace.
CN101575255A discloses a single-pass radiant coil including an inner tube and an outer tube, wherein feedstock flows along an annular gap formed between the inner tube and the outer tube. However, there is still a problem of a thermal expansion difference between the inner and outer tubes. Therefore, this kind of radiant coil has a poor mechanical property, and no significant process improvements compared with conventional single-pass radiant coils. Moreover, the cost for the single-pass radiant coil disclosed in CN101575255A is substantially high.